Method of making high-strength tin plate



Nov. 23, 1965 BUCKL/NG' STRENGTH, psi

H. E. HARTNER ETAL 3,219,494 METHOD OF MAKING HIGH-STRENGTH TIN PLATE Filed June 28, 1962 7.97 an sums, l07- POUND ans/s WEIGHT l A I U II II II x D U I! II If I I20 I k 1/0 I A .90 A: /l-. 80 fl/ 70 j CUMULATIVE PEI? CENT EFFECT OF NITRIC/NC 0N BUCKL/NC STRENGTH 0F CAN ENDS //'VV/V7'0/?5 HOWARD E. HART/VE/Pand ANDREW LES/V5)" Attorney United States Patent fifice 3,219,494 METHOD OF MAKING HIGH-STRENGTH TIN PLATE Howard E. Harmer, Pittsburgh, and Andrew Lesney,

Frazer Township, Allegheny County, Pa assignors to 'nited States Steel Corporation, a corporation of New Jersey Filed June 28, 1062, Ser. No. 205,926 2 Claims. (Cl. 148-46) This application is a continuation-in-part of our copending application, Serial No. 112,075, filed May 23, 1961, now abandoned.

This invention relates generally to nitrogen additions to steel, and more particularly, to adding nitrogen to a strip of steel and dififusing the added nitrogen through the steel.

Th conventional method of producing high nitrogen steel (above 0.010% N) is by ladle or mold additions of calcium cyanamid or similar nitrogen-bearing compounds. This method of adding nitrogen to the steel while it is in the molten state has several disadvantages. Such nitrogen-bearing compounds are expensive, the nitrogen recovery by the steel varies widely, smoke and fumes create a safety hazard, and rolling of high nitrogen steel is diificult with yield being comparatively low.

It is therefore a principal object of this invention to add nitrogen to steel while the steel is in the solid state.

A related object of this invention is to add nitrogen to steel after it has been rolled to a Strip.

Yet another object of this invention is to provide a method of closely controlling the amount of nitrogen added to steel.

A more particular object of this invention is to use ammonia gas as an agent to introduce nitrogen into steel, and thereafter cause uniform diffusion of the added nitrogen through the steel.

Other objects will appear, and a fuller understanding of the invention may be had by reference to the following specification and attached drawing.

The drawing is a graph showing the buckling strength of can ends formed from steel made according to this invention.

It has been found that if a steel strip i nitrided to produce a nitrogen-rich outer case, and thereafter heated to a temperature above the recrystallization temperature of the steel in an atmosphere fre of nitriding agents, the nitrogen will diffuse through the strip and a steel of superior stren th properties is produced.

This method of forming a high strength steel has particular application in, though not limited to, the manufacture of steel for tin plate, so the description will be of the process as it is applied to such coils of steel. One or more opened coils are placed in a furnace. This type of coil and how it is produced are described in an article in Industrial Heating, May 1958, p. 949. The composition range for a typical steel suitable for the method of this invention is given in Table I below. This is a conventional range for steel suitable for electrolytic tin plating. However, this range is illustrative only, and any steel which can be nitrided and the nitrogen diffused 3,219,494 Patented Nov. 23, 1965 through the steel uniformly can be treated according to this invention.

Table I Component: Percent by weight C 0.04 to 0.20

Mn 0.25 to 0.75

P 0.012 max.

S 0.040 max.

Si 0.010 max.

Cu 0.13 max. Ni 0.10 max.

Cr 0.06 max Mo 0.05 max Fe and residual impurities* Bal.

*The nitrogen content is about 0.003%, and in this type of steel can be considered a residual impurity; the Cr and Mo contents are residual amounts and hence, this steel is substantially free of strong nitride formers.

A cover i placed on the furnace and the coil or coils in the furnace are heated. Ammonia gas is introduced into the furnace at this early point in the heating cycle. Upon heating, the ammonia gas produces the nitrogen needed for nitriding by the following reaction:

The hot surface of the steel acts as a catalyst to decompose the ammonia and a portion of atomic nitrogen corning into contact with the surface is absorbed into the surface of the steel forming a nitrogen-rich outer case. The remainder of the atomic nitrogen reacts to form N The heating of the coils continues and the ammonia atmosphere is maintained until the desired amount of nitrogen is absorbed into the case. Producing this type of nitride case on steel is well known in the art. It is also known that different concentrations of ammonia gas and different temperatures, and the time the steel is maintained in a nitriding atmosphere affect the amount of nitrogen absorbed in the case and the depth of the case. Relatively simple experimentation will indicate the specific times, heating rates (i.e. temperatures) and concentrations of ammonia gas needed to provide a given percentage of nitrogen in the steel. It has been found generally that in adding between 0.015% and 0.15% nitrogen to steel by this method that the increase of nitrogen percentage is directly dependent upon the time, temperature, and concentration of the ammonia gas.

Th nitride case produced by this initial step is hard and brittle. Subsequent operations performed on a strip having a nitrided case tend to crack the steel. Further, this case does not impart much, if any, additional strength to the steel. Hence, it has been found to be necessary to diffuse the nitrogen from the case substantially uniformly through the steel.

To accomplish this diffusion the ammonia gas is purged from the furnace while the steel coils are continued to be heated by introducing a gas free of nitriding agents into the furnace. Conventional inert atmosphere gases, such as 5% H and 95% N can be used. The coils are heated in this atmosphere gas which is free of nitriding agents to above the recrystallization temperature of the steel and given a short soak. This heating to a temperature above the recrystallization temperature of the steel causes the nitrogen to diffuse from the nitrogen-rich case uniformly throughout the steel. This uniform diffusion is a diffusion of the nitrogen from the case to the core below the case. There may be some variation of the amount of nitrogen in the case at different locations on the strip. Hence, uniform as used herein means uniform through an entire cross section at any givenlocationoii the strip.

It has been found that for coils of steel to be tin plated, which steels are substantially free of strong nitride formers such as Cr, Mo and Al (i.e. there is substantially no detectable acid soluble Al and no more than about .05 Mo and .06% Cr and the total amount of strong nitride formers is less than .5%) that heating between 1050 F. and 1150" F. and holding at this temperature for one-half to three-quarters of an hour is optimum, although temperatures between 1000 F. and 1350 F. would be satisfactory. For other types of steel the optimum temperature ranges and time may vary but in all instances it is necessary to heat above the recrystallization temperature of the steel. The heating, from the be ginning, is continuous since commercially this is desired. However, it is necessary only to first form a nitrogenrich case, and then cause diffusion of the nitrogen from the case uniformly throughout the steel. Hence, the invention could be practiced in two separate steps without continuously heating the coils. For example, the coils could be held at a nitriding temperature for a period of time, and then heated to above the recrystallization temperature.

After the nitrogen has been diffused through the steel, the coils are cooled. The resulting structure is a high nitrogen steel which has excellent strength and does not have a hard or brittle case. The coils are then electrolytically plated with tin in a conventional manner.

Several tests were performed on can ends and test strips formed from coils of the composition of Table I and having nitrogen added and diffused according to this invention. The nitrogen content varied between 0.012% and 0.051%. Ninety tensile tests were made on steel from coils having a 107 pound basis weight. The tests showed a minimum tensile strength of 67,000 p.s.i. and a maximum of 117,400 p.s.i., with the median being about 79,000 p.s.i. Ten tensile tests were made on steel from coils having a 100 pound basis weight. The tests showed a minimum tensile strength of 71,000 p.s.i. and a maximum of 75,500 p.s.i., with the median being about 74,000 psi. Twenty tensile tests were made on the steel from coils having 90 pound basis weight. The tests showed a minimum tensile strength of 63,700 psi. and a maximum of 76,500 p.s.i., with the median being about 71,800 p.s.i.

Tests for buckling strength were performed on can ends formed from coils having the above basis weights. The cumulative percentages of the number of ends having buckling strength lower than a given value is indicated in the graph. This graph indicates that less than 1% of the ends formed from 107 pound basis weight coils had a buckling strength of less than 100 psi. and that only 2% of the ends formed from coils having a 100 pound basis weight had a buckling strength of less than 90 psi.

The above tensile strength and buckling strength values are much higher than any obtainable with the strip before nitriding and diffusing according to this invention. In fact these values compare favorably with those of high phosphorous steel which has been the conventional material used for beer-can-end application.

Metallographic examination of the resulting steel indicates that much of the nitrogen is in solid solution in ferrite. That which is not in solution has precipitated out as iron-nitride needles, Fe N and Fe N. The FGIGNQ needles are so small that they can be seen only with m g fi t n qb a nable with electron microscop s.

One of the outstanding advantages of this invention is the minimum change required in the normal annealing cycle. Conventional furnaces are used needing no modifications. During the early part of the cycle ammonia is substituted for conventional inert atmosphere gas. It may be necessary to modify slightly the heating rate of annealing temperature to obtain the desired percentage of nitrogen in the case. However, once this case has been attained, the NI-I is urged and the remainder of the annealing cycle is conventional including the use of conventional inert atmosphere gases. Ammonia gas is relatively inexpensive so there is no appreciable cost increase over normal annealing operations. Also, the nitrogen has been added to the steel after it has been rolled to the desired gage. Thus, difficulties encountered in rolling steel with high nitrogen content are eliminated. Closer control of the nitrogen content is possible than by ladle or mold additions, and no noxious fumes are admitted to the atmosphere.

Although one form of our invention has been described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. The method of making high-nitrogen steel strip for highstrength tin plate and the like, comprising:

(a) nitriding, after rolling, steel strip containing less than about 0.5% of strong nitride formers and no more than a residual amount of nitrogen, until a nitrogen-rich case is produced thereon which provides an average nitrogen content for the strip crosssection as a whole of above about 0.010%; and

(b) diffusing the nitrogen of said nitrogen-rich case substantially uniformly throughout the strip crosssection to obtain a high-strength high-nitrogen core, which does not have a hard case, said nitrogen diffusion being accomplished by annealing the strip at a temperature above its recrystallization temperature in an inert atmosphere.

2. The method of making high-nitrogen steel strip for high-strength tin plate and the like, comprising:

(a) nitriding, after rolling, steel strip containing less than about 0.5% of strong nitride formers and no more than a residual amount of nitrogen, until a nitrogen-rich case is produced thereon which provides an average nitrogen content for the strip crosssection as a whole of above about 0.010%, said nitriding being accomplished by heating the strip in opened coil form in an atmosphere of NH and (b) diffusing the nitrogen of said nitrogen-rich case substantially uniformly throughout the strip crosssection to obtain a high-strength high-nitrogen core, which does not have a hard case, said nitrogen diffusion being accomplished by annealing the strip in opened coil form at between 1000 F. and 1350 F. in an inert atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 1,774,999 9/1930 Sergeson 14816.6 2,592,282 4/1952 Hodil 148-l6 2,594,129 4/1952 Crego 148-16 3,058,856 10/1962 Miller 14816 FOREIGN PATENTS 522,252 6/ 1940 Great Britain.

BENJAMIN HENKIN, Primary Examiner.

WINSTON A. DOUGLAS, DAVID L. RECK,

Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,219,494 November 23, 1965 Howard E. Hartner et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 6, for "of" read to line 9, for

"urged" read purged Signed and sealed this 27th day of September 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

1. THE METHOD OF MAKING HIGH-NITROGEN STEEL STRIP FOR HIGH-STRENGTH TIN PLATE AND THE LIKE, COMPRISING: (A) NITRIDING, AFTER ROLLING, STEEL STRIP CONTAINING LESS THAN ABOUT 0.5% OF STRONG NITRIDE FORMERS AND NO MORE THAN A RESIDUAL AMOUNT OF NITROGEN, UNTIL A NITROGEN-RICH CASE IS PRODUCED THEREON WHICH PROVIDES AN AVERAGE NITROGEN CONTENT FOR THE STRIP CROSSSECTION AS A WHOLE OF ABOVE ABOUT 0.010%; AND (B) DIFFUSING THE NITROGEN OF SAID NITROGEN-RICH CASE 